This invention is an improvement in the structure adapted to contain a pool of molten metal on which molten glass is floated as it is being formed into a flat glass ribbon in the float process. In particular, it involves a structure that is capable of maintaining integrity at higher temperatures than are customary in the float process.
A float glass forming chamber typically includes a steel casing lined with ceramic refractory material of sufficient thickness to provide a temperature gradient that places the exterior side of the refractory at a temperature below the melting temperature of the molten metal, which is usually tin or an alloy thereof. The object is to prevent the migration of molten tin into contact with the steel casing which would be readily attacked by the molten tin. The refractory lining takes the form of preformed and prefired blocks bolted to the bottom of the casing by bolts or the like as shown in U.S. Pat. Nos. 3,584,477 (Hainsfurther), 3,652,251 (Brichard), and 3,669,640 (Brichard et al.), or a layer cast in place around anchors affixed to the casing as shown in U.S. Pat. No. 3,594,147 (Galey et al.). The mechanical attachments of the refractory lining to the casing are required to prevent the buoyant force of the molten tin from lifting the refractory out of place since the specific gravity of the tin is much greater than that of the refractory.
Avoiding the use of individual bolts or anchors would be advantageous in that it eliminates several potential sources of failure. Bolt holes in some arrangements need to be filled with a separate refractory material that can come loose and interfere with the forming process. Attack of tin on the bolts or anchors can be a source of bubbles that degrade the quality of the glass. Mechanical attachments can also be the source of cracks, particularly at higher temperatures. Moreover, refractories that are suited for higher temperature applications tend to be more brittle and therefore can be more easily fractured by the localized stresses produced by bolts or anchors.
An arrangement without direct mechanical attachment of the refractory bottom blocks to the casing is shown in U.S. Pat. No. 3,427,142 (De Lajarte). There, wedge shaped center blocks are held in place by the adjacent blocks, and the outer blocks are restrained vertically by an extension of the metal casing over the tops of the outer blocks. However, the wedged block arrangement of the patent has been found to be inadequate as an alternative to the usual mechanical attachments such as bolts or anchors. This is because the very large buoyant force of the small amount of molten tin that penetrates between the blocks is sufficient to lift the center blocks, opening gaps therebetween that provide enlarged paths for the molten tin to attack the casing and to lift the blocks further. Additionally, the structure disclosed in that patent can provide a proper, tightly wedged fit at only one temperature since the thermal expansion and contraction that accompany normal temperature variations during operation would not be accommodated, much less the large temperature changes involved in starting up or shutting down the operation. If the casing and blocks are sized to fit at peak operating temperatures, gaps will open and the blocks will lift when production interruptions cause the temperature to drop. On the other hand, sizing the casing and blocks for a lower temperature will cause excess stress and possible cracking of the blocks, particularly blocks of high temperature durability refractory such as high alumina refractories.
Processes that put exceptional stress on the molten tin containment structure are disclosed in U.S. Pat. Nos. 4,741,749 (Sensi et al.) and 4,744,809 (Pecoraro et al.). In these processes the molten glass is deposited onto the molten tin at temperatures substantially higher than in a conventional float process. Tin temperatures where the glass is first received onto the tin may be as high as 2300.degree. F. (1260.degree. C.), which may be compared to a conventional delivery region tin temperature of about 1900.degree. F. (1040.degree. C.). At conventional temperatures the tin containing vessel may be made of clay type refractories containing alumina and silica as disclosed in U.S. Pat. No. 3,594,148 (Smith et al.). But at higher temperatures the clay type of refractory is subject to accelerated attack by alkalis present, which can lead to bloating, shelling, and delamination of the bottom refractories. At the higher temperatures, using more suitable refractories such as high alumina refractories would be preferred as taught by U.S. Pat. No. 3,669,640 (Brichard et al.), but this type of refractory is more brittle and subject to fracture when subjected to mechanical stress. The usual attachment means can lead to stress being applied to the refractory as temperatures change, thereby increasing the likelihood of fracture. The extra refractory thickness and the large temperature gradient required by a high temperature operation also increase the amount of stress that may be created in the refractory. Therefore, the use of refractories with suitable chemical durability for the high temperature environment heretofore entailed accepting a high risk of bottom failure due to refractory fracture.